Commutation phenomena in electrical machines. Part 1: Basic experiments in flashover

Work on an experimental turntable rig built to simulate a commutator has demonstrated that, as a direct voltage between the two brush arms is increased, (a) the tendency to form short arcs from a brush to the segment leaving the brush increases and (b) the persistence of the associated bar-to-bar arcs increase suddenly. Invariably, the intersegmental arcs are initiated at the positive brush. Once a continuous series of intersegmental (bar-to-bar) arcs exists between the brush arms, a flashover takes place. High-speed ciné photography has shown also that the intersegmental arcs increase rapidly in intensity and that the discharges are propelled into the air above the surface of the commutator to a distance of about 1 in by a process known as `jetting¿. Under these conditions, the arc current is increasing extremely rapidly, producing a mass of ionised gas above the commutator. This behaviour, and the damage produced, is consistent with features noted on large machines after flashover, and it is likely that this mechanism of flashover applies to such machines also. Having established that the major flashover mechanism in the experimental rig involves a continuous string of intersegmental arcs, the basic properties of such arcs and means of extinguishing them rapidly were investigated. Clearly flashover can be prevented either by restricting the formation of short arcs or by extinguishing them quickly. Two simple mechanisms of extinguishing the arcs have been studied: an electromagnetic method which propels the arc rapidly along the gap to the edge of the commutator where it is extended and extinguished and a method using an airblast.